The invention relates to a device for driving an air-conditioning compressor having an electric motor, in particular for motor vehicles.
Various requirements, in particular reliability, low costs, low maintenance need and the lowest weight possible, have to satisfied when driving air-conditioning compressors having electric motors. Furthermore, with the effect of saving on energy, the air-conditioning compressor should only be switched on when it is being used. However, when started repeatedly, the air-conditioning compressor operates counter to the pressure of the refrigerant, with the result that starting of the electric motor is made more difficult and, in particular, a high starting current arises.
Starting is more difficult particularly in sensorless activation methods where the rotor position is obtained from the electromotive force which, because of lack of speed, is virtually impossible to measure.
The object of the present invention is to specify a drive for air-conditioning compressors with the abovementioned disadvantages being avoided. According to the invention, this object is achieved in that an arrangement is provided for reducing the difference in pressure between the intake side and delivery side of the air-conditioning compressor during a starting phase of the electric motor.
The invention has the advantage that the starting current is substantially reduced and the electric motor does not therefore have to be configured to deal with the correspondingly high load. When spiral compressors are used, the cooling capacity is then more rapidly available because of the more rapid starting, since said spiral compressors seal better at higher speeds. In addition, the power lost in an electronic control device turns out to be less on account of the lower starting current.
Particularly if the refrigerant is also used for cooling the control device, the cooling of the control device is more rapidly available, which likewise permits use of cost-effective transistors. Finally, there is a lower load on the vehicle electric system during starting.
Demagnetization may also be prevented without an electronic control device, for example in the case of a DC motor containing brushes.
In a first advantageous refinement of the invention, provision is made for the arrangement for reducing the difference in pressure to be formed by an expansion valve which is arranged in the refrigerant circuit being controllable.
A second advantageous embodiment consists in that the arrangement for reducing the difference in pressure is formed by a controllable bypass between the intake side and delivery side of the air-conditioning compressor. In this case, the bypass can be arranged within or outside the housing of the air-conditioning compressor and can be controlled either electrically or mechanically.
A third advantageous embodiment makes provision for the arrangement for reducing the difference in pressure to be formed by a controllable, additional expansion valve which is arranged in the refrigerant circuit between the high-pressure side of the expansion valve and the intake side of the air-conditioning compressor.
A fourth advantageous embodiment consists in that the arrangement for reducing the difference in pressure is formed by a speed-controlled valve which is arranged in the air-conditioning compressor. In this case, provision may preferably be made for the speed-controlled valve to be a centrifugal seal.
The lower load during starting makes possible a development which consists in that the electric motor is a sensorless motor having electronic commutation. The technical outlay is reduced thereby, on the one hand, while the reliability is increased, on the other hand.
A further advantageous refinement of the invention consists in that a control unit activates the arrangement for reducing the difference in pressure within a predetermined period of time after the electric motor is switched on, and afterwards switches it off again.
In another advantageous refinement, provision is made for the controlling means of the arrangement for reducing the difference in pressure to be controllable as a function of the speed of the compressor.
Since the reduction in the difference in pressure leads to cooling of the intake side, it is additionally advantageous if the control unit and the electric motor are in heat-conducting connection with the refrigerant on the intake side.